1. Field of the Invention
The present invention relates to a pressure detection apparatus comprising a pressure sensor including a pressure detection element having a bridge circuit constituted by a pressure sensitive resistance to measure a pressure of gas or fluid.
2. Description of the Related Art
FIG. 7 is a sectional view showing a pressure detection apparatus generally used as, e.g. . . , an automobile intake pressure apparatus. Referring to FIG. 7, a pressure sensor 31 (to be described later) is mounted on a circuit substrate 30. The pressure sensor 31 is mounted such that a lead 31a projecting from one side of the pressure sensor 31 is soldered on the circuit substrate 30. A ceramic circuit substrate 32 having a U-shaped section is arranged on the circuit substrate 30 to cover the pressure sensor 31. An amplification circuit body 33 for amplifying an output signal from the pressure sensor 31 is disposed inside the ceramic circuit substrate 32. An adjustment resistor (not shown) constituting an amplfilcation circuit together with the amplification circuit body 33 is formed outside the ceramic circuit substrate 32. The adjustment resistor is printed on the outer surface of the ceramic circuit substrate 32. The pressure sensor 31, the amplification circuit body 33, and the adjustment resistor are electrically connected to each other with a wire 32a disposed on the ceramic circuit substrate 32 and a printed wire (not shown) disposed on the circuit substrate 30.
The pressure sensor 31 has a cylindrical nipple 31b for supplying a pressure of an object to be measured such as air. The pressure sensor 31 is mounted such that the cylindrical nipple 31b passes through a through hole (not shown) formed in the circuit substrate 30. Further, on the circuit substrate 30, a terminal 34 for leading out an signal output from the pressure sensor 31 and amplified by the above amplification circuit is disposed. An assembly constituted by the circuit substrate 30, the pressure sensor 31, and the ceramic circuit substrate 32 is fixed on a sensor accommodation vessel 35 such that the peripheral portion of the circuit substrate 30 is supported by the sensor accommodation vessel 35. The assembly is covered with a sensor accommodation vessel 36 to be accommodated in two sensor accommodation vessels 35 and 36. The pressure sensor 31 and the ceramic circuit substrate 32 are accommodated in the space formed inside the sensor accommodation vessels 35 and 36 not to be in contact with the vessels. A connector portion 36a to which an external connector (not shown) is connected is formed in the sensor accommodation vessel 36. The terminal 34 is supported at the center of the connector portion 36a to be buried therein. In the sensor accommodation vessel 35, a pressure supply tube 35a communicating with a cylindrical nipple 31b is formed.
FIG. 8 is a sectional view showing the internal structure of the pressure sensor 31. Referring FIG. 8, a pressure detection element 40 having a bridge circuit (to be described later) formed therein is airtightly disposed in the pressure sensor 31 to seal one end of the cylindrical nipple 31b. The pressure sensor 31 is accommodated in a can package 41 which is airtightly sealed. The pressure sensor 31 is electrically connected to the lead 31a with a bonding wire 42.
FIG. 9 is an upper view showing the pressure detection element 40. Referring to FIG. 9, the pressure detection element 40 consists of, e.g. . . , silicon single-crystal. The central portion of the element 40 is partially formed to have a small thickness, and a diaphragm portion 43 deflected by a pressure is formed in the pressure detection element 40. A plurality of gauge resistor are formed at the peripheral portion of the diaphragm portion 43, and these gauge resistors are connected to each other to constitute a bridge circuit 44. The bridge circuit 44 converts deflection of the diaphragm portion 43 into an electrical change.
The operation of the apparatus will be described below. An object to be measured is guided through the pressure supply tube 35a disposed in the sensor accommodation vessel 35 and passes through the cylindrical nipple 31b to reach the pressure detection element 40. In accordance with this, the bridge circuit 44 disposed around the diaphragm portion 43 converts the deflection into an electrical change. An output from the pressure detection element 40 reaches the amplification circuit body 33 through the bonding wire 42, the lead 31a, and the circuit substrate 30 to be amplified by the amplification circuit body 33. The amplified output passes through the circuit substrate 30, and is output outside the sensor accommodation vessels 35 and 36 through the terminal 34.
In the pressure detection apparatus with such an arrangement, the output characteristics of the pressure detection apparatus are changed by function trimming performed such that the adjustment resistor formed outside the ceramic circuit substrate 32 is trimmed by a laser beam. More specifically, the adjustment resistor formed outside the ceramic circuit substrate 32 is partially trimmed by a laser beam to change a resistance value, thereby changing the output characteristics.
The range of a pressure which is received by the pressure detection apparatus is changed by the following manner. That is, the diaphragm portion 43 in the same pressure detection element 40 is changed in thickness, and the resistance value is changed accordingly.
In the conventional pressure detection apparatus described above, the adjustment resistor for changing characteristics is formed on the ceramic circuit substrate 32 by printing. The adjustment resistor is flatly formed by using one material or few types of materials, and different types of resistors are manufactured by making the areas of the resistors different. For this reason, the ceramic circuit substrate 32 requires a large area to form different types of resistors. Since the area of the ceramic circuit substrate 32 is limited to a specific area, the range of adjustable resistance value is limited to a specific range. For this reason, in order to considerably change characteristics, the ceramic circuit substrate 32 must be changed in design to be remade.
Furthermore, even if the resistance value is within the adjustment allowance range of the adjustment resistor, when an amount of trimming for adjustment is large, the trimming requires a long time. As a result, a time for changing the output characteristics is prolonged disadvantageously.
When the range of pressure which is received by the pressure detection apparatus is changed, the resistance value must be largely changed. For this reason, the ceramic circuit substrate 32 must be changed in design to be remade.